When power conversion is performed using a full-bridge circuit, an isolated power supply is connected to input points of the full-bridge circuit, a load or the like is connected to output points of the full-bridge circuit, and operation is performed in a floating potential condition without grounding these input and output points (as in motor driving or the like, for example). This is called floating operation.
FIG. 5 is an explanatory diagram showing connection of a conventional full-bridge power converter used in floating condition. In this diagram, an isolated power supply 102 is connected to the input side and a load 103 is connected to the output side of a full-bridge power converter 101. Further, an input capacitor 111 is connected between the two input terminals of the full-bridge power converter 101.
A full-bridge circuit 110 is equipped with four switching elements S1˜S4. The switching element S1 and switching element S2 are series connected, and the switching element S3 and switching element S4 are series connected. The isolated power supply 102 is a power source that outputs DC power, its power output terminals are adapted for isolated output, and Y capacitors Cy are connected between these output terminals and a ground is provided between the Y capacitors Cy.
A connection point between the switching element S1 and switching element S3 is connected to one input terminal of the full-bridge power converter 101.
A connection point between the switching element S2 and switching element S4 is connected to another input terminal of the full-bridge power converter 101.
Further, one end of an inductor 112 is connected to a connection point between the switching element S1 and switching element S2, and one end of an inductor 113 is connected to a connection point between the switching element S3 and the switching element S4.
An output capacitor 114 is connected between another end of the inductor 112 and another end of the inductor 113, and the output terminals of the full-bridge power converter 101 are connected across this output capacitor 114.
The switching operation of the switching elements S1 to S4 of the full-bridge circuit 110 is controlled by an unshown control unit to operate so that the switching elements S2 and S3 are turned OFF when the switching elements S1 and S4 are turned ON and the switching elements S1 and S4 are turned OFF when the switching elements S2 and S3 are turned ON.
When DC voltage is supplied from the isolated power supply 102, the full-bridge power converter 101 outputs power to the load 103 in a floating potential condition. At this time, the switching elements of the full-bridge circuit 110 repeat ON/OFF operation to make it possible to control the values and polarity of the voltage and current output to the load 103 by this switching operation.
Moreover, when performing power conversion in common mode, there are cases in which, as described in patent reference 1, for example, power output from a bridge circuit is output to a load or the like through a transformer whose primary winding and secondary winding are isolated. A known practice is to interpose a transformer or the like in this manner so as to output power lowered in common-mode noise attributable to a bridge circuit or the like.